Add markdownlint (#20512)

Add `markdownlint` linter and fix issues. Config is based on the one
from electron's repo with a few rules relaxed.

.markdownlint.yaml

@@ -0,0 +1,18 @@
+commands-show-output: false
+fenced-code-language: false
+first-line-h1: false
+header-increment: false
+line-length: {code_blocks: false, tables: false, stern: true, line_length: -1}
+no-alt-text: false
+no-bare-urls: false
+no-blanks-blockquote: false
+no-duplicate-header: {allow_different_nesting: true}
+no-emphasis-as-header: false
+no-empty-links: false
+no-hard-tabs: {code_blocks: false}
+no-inline-html: false
+no-space-in-code: false
+no-space-in-emphasis: false
+no-trailing-punctuation: false
+no-trailing-spaces: {br_spaces: 0}
+single-h1: false